1. Field of the Invention
The present invention relates to a structure of a low frequency surface plasmon polariton (SPP) waveguide, and in particular to a novel leakage waveguide composed of a unique metallic hollow block periodic structure surface that enables the metal surface to transmit electromagnetic waves in a more efficient manner and may serve as a highly directional radiation element.
2. The Related Arts
Using the general idea of surface plasmon polariton to manipulate electromagnetic waves in order to make transmission along a sub-wavelength path possible is an important contemporary issue for effectively enhancing device density of a photo circuit system and an integrated circuit. Surface plasmon polaritons (SPPs) are a hybridized excitation state of electron and photon occurring at interface between a metal and a medium (usually air). SPPs have a field amplitude that is maximum at the interface and exponentially attenuates in the metal and the medium. SPPs provides a possibility of guiding electromagnetic beyond diffraction limit and thus attract wide interesting in the surface plasmon polariton photonics.
Due to the electromagnetic field being highly confined around the interface between the metal and the medium, SPPs has an inherent two-dimensional (that is surface) transmission property that makes it a priority candidate for high integration of integrated optical circuit and design of photon device in the range of subwavelength. It is thus urgently desired to transmit signals in a circuit system of a relatively low frequency and to promote the application of the idea of SPPs to frequency bands of even lower frequencies, such as terahertz bands and microwave bands, for transmission of signals
In general, metals have a SPPs frequency that is in the ultraviolet band so that the behavior of the metals in the terahertz band is close to perfect electric conductor (PEC). This makes SPPs poor in confinement of electromagnetic field on the metal surface, incapable of effectively concentrating electromagnetic wave, thereby limiting the application of a leakage antenna of a regular structure in the terahertz band and the microwave band.
However, since the SPPs mode is generally impossible to realize in a low frequency, such as the microwave and terahertz bands, to achieve a similar physical phenomenon in a low frequency band, periodic metallic blocks or periodic grooves in a high density arrangement may be formed on the metal to realize high confinement of electromagnetic field. In a waveguide structure used in the prior art, a plurality of unit cell blocks that is spaced from each other by a fixed distance is arranged on a top surface of a planar metal substrate. In such a periodic structure of solid metallic blocks, the distribution of an electromagnetic field is highly confined in-between two adjacent unit cell blocks.